Color LCDs have become key components in portable electronic devices. In particular, users of mobile telephones, portable electronic games, personal digital assistants (PDAs), and other handheld electronic devices expect the display performance of their portable electronic device to be similar to that of a laptop personal computer in the backlight mode with respect to characteristics such as brightness and color saturation. Additionally, users expect the displays for portable electronic devices to maintain good readability in both indoor and outdoor environments, including those with high ambient light. Thus, transflective LCDs exist that operate in both a transmissive (backlight) mode and a reflective (ambient light) mode.
Throughout this specification, “front” shall refer to portions of an LCD that are closer to a viewer and “rear” shall refer to portions of an LCD that are farther away from a viewer. In the drawings, elements that are in the “front” are visually above elements that are in the “rear.”
FIG. 3 shows a related art transflective color LCD 300 with an internal reflector 335. A front substrate 310, usually constructed of glass, has a color filter 312 on a rear surface and a front polarizer 360 on a front surface. A rear substrate 320, usually constructed of glass, has an internal reflector 335 formed on a front surface and a rear polarizer 370 on a rear surface. Between the front substrate 310 with associated elements 360, 312 and the rear substrate 320 with associated elements 335, 370, lies a liquid crystal layer 350 sandwiched between a front transparent electrode 340 and a rear transparent electrode 330. Furthermore, a backlight sub-system 380 is disposed behind the rear polarizer 370 and functions as a light source during the transmissive mode.
Arrow 393 demonstrates the operating principles of the transflective color LCD 300 with an internal reflector 335 during the transmissive mode. Light generated from the backlight sub-system 380 passes through the rear polarizer 370, the rear substrate 320, the internal reflector 335, the rear transparent electrode 330, the liquid crystal layer 350, the front transparent electrode 340, the color filter 312, the front substrate 310, and the front polarizer 360. Arrow 391 demonstrates the operating principles of the transflective color LCD 300 with an internal reflector 335 during the reflective mode. Ambient light passes through the front polarizer 360, the front substrate 310, the color filter 312, the front transparent electrode 340, the liquid crystal layer 350, the rear transparent electrode 330, reflects on the surface of internal reflector 335, redirects toward the front substrate 310, and passes back through the front polarizer 360.
In this transflective color LCD 300 with an internal reflector 335, an optical path in the reflective mode, as indicated by arrow 391, differs significantly from an optical path in the transmissive mode, as indicated by arrow 393. These differing optical paths affect several display characteristics noticeable to the user, such as color saturation and brightness. For example, color purity is affected by the fact that light rays pass through the color filter 312 twice in the reflective mode and only once in the transmissive mode.
To rectify this situation, there are transflective color LCDs with a dual cell gap construction. For example as shown in FIG. 4, the color filter 412 is varied so that there are reflective regions with lower dye density color filtering and transmissive regions with higher dye density color filtering. Then, the internal reflector 435 is patterned and aligned with the reflective and transmissive regions of the color filter so that lights rays are reflected through thinner liquid crystal regions and transmitted through thicker liquid crystal regions. Finally, the transition areas between the reflective regions and transmissive regions are masked to reduce light leakage.
The remainder of the transflective color LCD 400 is similar to that shown in FIG. 3. A front substrate 410, usually constructed of glass, has a front transparent electrode 440 and a color filter 412 on a rear surface and a front polarizer 460 on a front surface. A rear substrate 420, usually constructed of glass, has a patterned internal reflector 435 formed on a front surface and a rear polarizer 470 on a rear surface. Between the front substrate 410 with associated elements 460, 440, 412 and the rear substrate 420 with associated elements 435, 470, lies a varied liquid crystal layer 450 sandwiched between the front color filter 412 and a rear transparent electrode 430. Furthermore, a backlight sub-system 480 is disposed behind the rear polarizer 470 and functions as a light source during the transmissive mode.
Dual-cell gap construction compensates for the differing optical paths in the transflective color LCD 400 with an internal reflector 435 but results in a complicated manufacturing process, including additional mask and process steps, which results in a higher manufacturing cost. Additionally, the masking itself causes lower brightness for the transflective color LCD 400.
We turn now to another transflective color LCD that does not have the optical path problem of the transflective color LCD 300 with an internal reflector, nor the complicated manufacturing process of the transflective color LCD 400 with a dual cell gap construction, but instead has a parallax problem. FIG. 5 shows a related art transflective color LCD 500 with an external reflector 575. A front substrate 510, usually constructed of glass, has a color filter 512 on a rear surface and a front polarizer 560 on a front surface. A rear substrate 520, usually constructed of glass, has a rear polarizer 570 on a rear surface and an external reflector 575 behind the rear polarizer 570. Between the front substrate 510 with associated elements 560, 512 and the rear substrate 520 with associated elements 570, 575 lies a liquid crystal layer 550 sandwiched between a front transparent electrode 540 and a rear transparent electrode 530. Furthermore, a backlight sub-system 580 is disposed behind the external reflector 575 and functions as a light source in the transmissive mode.
Arrow 593 demonstrates the operating principles of the transflective color LCD 500 with an external reflector 575 during the transmissive mode. Light generated from the backlight sub-system 580 passes through the external reflector 575, the rear polarizer 570, the rear substrate 520, the rear transparent electrode 530, the liquid crystal layer 550, the front transparent electrode 540, the color filter 512, the front substrate 510, and the front polarizer 560. Arrows 591, 592 demonstrate the operating principles of the transflective color LCD 500 with an external reflector 575 during the reflective mode. As shown by arrow 591, ambient light passes through the front polarizer 560, the front substrate 510, the color filter 512, the front transparent electrode 540, the liquid crystal layer 550, the rear transparent electrode 530, the rear substrate 520, and the rear polarizer 570. Then, as shown by arrow 592, the light rays are reflected by the external reflector 575 and redirected back through the rear polarizer 570, the rear substrate 520, the transparent electrode 530, the liquid crystal layer 550, the front transparent electrode 540, the color filter 512, the front substrate 510, and the front polarizer 560.
Arrow 592 indicates the optical path in the reflective mode, while arrow 593 indicates the optical path in the transmissive mode. Note that arrow 591 is not included in the reflective optical path because the rear polarizer 570 resets the optical effects of the light rays. As shown in FIG. 5, there is little difference in the two optical paths. There is, however, a parallax problem due to the significant distance 596 between the external reflector 575 and the liquid crystal layer 550 given that the rear substrate 520 has a thickness of approximately 500 micrometers and the rear polarizer has a thickness of approximately 100 micrometers. Thus, in the reflective mode, the pixel on the incoming light rays shown by arrow 591 may be different from the pixel on the outgoing light rays shown by arrow 592. This becomes more of a problem as pixels get smaller, which is the trend in LCDs and especially color LCDs.
Thus, there is a need for a transflective color LCD that avoids the problems of significantly differing optical paths and complicated construction but does not have the parallax problems caused by significant distance between an image-forming layer (e.g., a liquid crystal layer) and a reflector. The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Drawings and accompanying Detailed Description.